Sen.
Rand Paul
hasn't said whether he will seek the 2016 GOP presidential nomination. But his aggressive groundwork seems to point to no other outcome.

In recent weeks, the Kentucky Republican announced political hires in quick succession in Iowa, New Hampshire and Michigan— states key to winning his party's nomination. Staffers mention a future campaign headquarters in Louisville and claim an email list of one million supporters, details most potential presidential hopefuls keep quiet. A super PAC launched by backers shortly before the 2012 election offers a repository for big donors.

On Monday, Mr. Paul begins a three-day, 10-stop swing through Iowa, marking his 10th visit in this election cycle to one of the first three states on the traditional nominating calendar. Only Sen.
Ted Cruz
(R., Texas) has made more trips, at 11.

The early spade work serves an important purpose for Mr. Paul, a 51-year-old tea-party favorite elected to the Senate less than four years ago. As a central figure in a movement at odds with the political establishment and an apostate from his party's muscular stance on foreign policy, Mr. Paul must prove that he has broader national support than his father, former Rep.
Ron Paul,
a three-time presidential candidate who never won a nominating contest.

"Rand Paul is for real—a 100%, dead-serious contender, and anyone who underestimates him should have his head examined," said
Phil Musser,
a Republican strategist who advised GOP presidential hopefuls
Tim Pawlenty
and
Mitt Romney
but is unaffiliated with any potential 2016 candidates. "He has changed the perception that a Paul could never win the nomination."

Asked about a candidacy, Mr. Paul said in June: "I would just say it's probably not a conclusion yet, but it is something that's an ongoing discussion."

Of course, early organizing doesn't guarantee electoral success. Other candidates who hired staff and traveled early but failed to win the nomination include
John McCain
in 2000 and Mr. Pawlenty, a former governor of Minnesota, in 2012.

As a central figure in a movement at odds with the political establishment, Mr. Paul must prove that he has broader national support than his father.
European Pressphoto Agency

Also in the 2012 cycle, former Sen.
Rick Santorum
began giving speeches to religious and political groups and scheduling appearances in Iowa in the fall of 2009, about three years before the general election, but didn't get the nod.

GOP political consultant
Curt Anderson,
who advises Louisiana Gov.
Bobby Jindal,
a potential rival to Mr. Paul, called the pre-campaign posturing a "joke" that is taken too seriously by the media. "You have to run the race that makes sense for you, not the one that will make the pundits think you're serious because you have an office in South Carolina," Mr. Anderson said.

Roughly a dozen Republicans are laying the groundwork for possible presidential bids in 2016, each with his own calculus and timetable. Rep.
Paul Ryan
of Wisconsin, the 2012 vice presidential nominee, hasn't ruled out running, but other potential candidates say he appears more interested in serving in senior House positions. Wisconsin Gov.
Scott Walker
is primarily engaged in winning re-election in November. Former Florida Gov.
Jeb Bush,
perhaps conflicted about a presidential bid, mostly conducts politics behind the scenes.

Mr. Paul's candidacy, by contrast, is taken as fact. On Saturday, Senate Minority Leader
Mitch McConnell
(R., Ky.) unabashedly linked Mr. Paul with another native son of their state who sought the White House, calling Mr. Paul "the most credible candidate for president of the United States since
Henry Clay.
"

Tim Albrecht,
a GOP strategist who used to work for Iowa Gov.
Terry Branstad
and isn't yet working for any 2016 presidential candidate, said Mr. Paul is "doing away with the formalities of saying, 'I'm not really looking,' and it's kind of refreshing."

By contrast, New Jersey Gov.
Chris Christie
was dismissive earlier this month when asked if he could still run for president in the aftermath of the scandal surrounding last year's George Washington Bridge traffic jam. "Who knows? I don't know," he told reporters.

Sen. Paul's inner circle includes about a half-dozen veterans of his Senate campaign and the presidential campaign of his father. Sen. Paul's leadership PAC has hired three veterans of presidential campaigns in Iowa, New Hampshire and Michigan, a step toward a national campaign that no other 2016 hopeful has taken. "We're trying to have a basic infrastructure in place before the general election, so if he formally announces after November we're ready to go," said
Steve Grubbs,
a former Iowa Republican Party chairman running Mr. Paul's leadership PAC in that state.

His counterpart in New Hampshire, GOP strategist
Mike Biundo,
called what he is doing "intel work"—culling information about the environment and political players that could be parlayed into a statewide campaign. He is setting up fundraisers for the New Hampshire Republican Party and House Republicans, as well as reaching out to voters who helped Mr. Paul's father come in second in the state's 2012 primary.

Sen. Paul has raised $7.8 million in the past 18 months through his re-election account, leadership PAC and a super PAC run by supporters,a required support mechanism for modern-day national campaigns. Among potential rivals who also have campaign accounts and PACs, the top fundraiser is Mr. Ryan, a member of the House leadership team who has collected $9.2 million.

The elder Mr. Paul was viewed as the leader of an ideological movement that motivated volunteers to advocate for limited government outside of his bid for the White House.

Of the $5.1 million spent since 2013 by Sen. Paul's campaign, leadership PAC and the super PAC, $3.3 million went to fundraising and consulting, which includes salaries, polling, research, voter targeting, direct mail and telemarketing, according to a Wall Street Journal analysis of data by the Center for Responsive Politics.

The biggest spender among possible presidential candidates with PACs is Mr. Ryan, who has paid out $7.2 million since 2013. Up for re-election to the House this year, he spent $3.3 million on fundraising and consulting and $250,000 on media and ads.

Mr. Ryan also gave away the most money, $730,000, to other candidates and party committees through his leadership PAC. In contrast, Mr. Paul's leadership PAC has given $150,000 to candidates and committees, while the super PAC run by his supporters spent $32,000 backing failed North Carolina Senate candidate
Greg Brannon.

Florida Sen.
Marco Rubio
is also building a political machine that could fuel a presidential bid, spending $1.9 million on fundraising and consulting through his leadership PAC and campaign account. He gave $148,000 to candidates and committees while spending an additional $400,000 on other candidates' behalf.

More than anyone in the 2016 field, Mr. Paul raises issues and seeks audiences guaranteed to draw national attention. Most recently, he called for returning voting rights to felons and expunging the records of some nonviolent drug offenders, presenting the ideas at the National Urban League conference in Cincinnati one week ago.

But Mr. Paul's efforts to act as a bridge between the tea party and the Republican establishment, while at the same time reaching out to minorities and young people, may leave a wide but shallow river of support, said Republican consultant
Hogan Gidley,
who worked for former presidential candidates
Mike Huckabee
and Mr. Santorum.

"Someone else will come along who is more polished and makes a better presentation, and there's no guarantee Mr. Paul will keep the voters he's reaching out to, if he ever had them in the first place," Mr. Gidley said. "The election is a long way off."

Corrections & Amplifications

An earlier version of this article incorrectly identified Mitch McConnell as Senate majority leader. He is Senate minority leader. It also said former Rep. Ron Paul was a two-time presidential candidate. He ran three times.

At least two Kentuckians, John Breckinridge and Alben Barkley, have run for president since Henry Clay did so in 1844. A previous version of this article incorrectly said Sen. Clay was the last Kentuckian to seek the White House.